Dienophiles, 2 -oxazolones

The checkers obtained erratic results in this step, possibly because of surface effects or trace impurities in the pressure vessel. In two other runs, only 16.8-18.8 g of crude product were obtained. In one case, high boiling oligomers were formed, but none of the desired product was produced. Impurities in the diene or dienophile did not appear to be the problem since runs which employed recrystallized 3-acetyl-2(3H)-oxazolone and redistilled 2,3-dimethyl butadiene also gave variable results. 

Avenoza A., Busto J. H., Cativiela C., Peregrina G. M. and Zurbano M. M. The use of 4-Hetaryliden- and 4-Aryliden-5(4H)-Oxazolones as Dienophiles in the Diels-Alder Reactions in Targets in Heterocyclic Systems-Chemistry and Properties vol 3. 1999, Eds. Attanasi O. A. and Spinelli D., Pb. Soc. Chim. Ital. Keywords hetero-Diels-Alder reactions... 

The 3-acyl-2(3F/)-oxazolones function as good dienophiles in cycloaddition reactions with cyclic 2,4-dienes such as cyclopentadienes and anthracenes. Thus, the thermal reaction of 3-acetyl-2(37/)-oxazolone with cyclopentadiene and the hexachloro and hexamethyl derivatives gives endo-cycloadducts exclusively. In particular, the chiral cycloadducts 221 and 223 derived from the diastereoselective Diels-Alder reactions of 3-(2-exo-alkoxy-l-apocamphanecarbonyl)-2(3/7)-oxazo-lones with hexamethylcyclopentadiene and 9,10-dimethylanthracene, respectively, are highly useful as chiral 2-oxazolidinone auxiliaries. The conformationally rigid roofed structures play a crucial role in affording excellent chiral induction (Fig. 5.54). 

Irradiation of 4-(but-3-enylidene)-2-phenyl-5(477)-oxazolone 733 leads to the [4 + 2] adduct 734 derived from 733 acting as both a diene and a dienophile in a solid-state photo Diels-Alder reaction (Scheme 7.226). °... 

Diels-Alder reactions of 4-heteromethylene-5(4/7)-oxazolones have been described. ( )-4-(Chloromethylene)-5(4//)-oxazolone 737 reacts with 2,3-dunethyl-butadiene in the presence of ethylaluminum dichloride to afford the cycloadduct 738. The cycloaddition reaction is characterized by high diastereoselectivity and occurs without appreciable isomerization of the dienophile. Further synthetic transformations of 738 yield 1-amino-3,4-dimethyl-6-hydroxy-cyclohex-3-enecar-boxylic acid 739 (Scheme 1.121) Examples of Diels-Alder reactions of acyclic dienes and unsaturated 5(4//)-oxazolones are shown in Table 7.50 (Fig. 7.61). 

Z)-4-[(5)-2,2-Dimethyl-l,3-dioxolan -ylmethylene]-2-phenyl-5(47/)-oxazolone 632 can react as a dienophile in diastereoselective Diels-Alder reactions. Thus, 632 undergoes a thermally induced Diels-Alder reaction with cyclic dienes, for example, cyclopentadiene and cyclohexadiene, to afford a mixture of the four... 

The chiral (E)-oxazolone derived from 1,2-O-isopropyIidene-D-glyceraldehyde has also been used as a dienophile in the Diels-Alder reaction and, in this case, ( /Z) isomerization of the oxazolone can be avoided using heterogeneous catalysts that promote the synthesis of trans-adducts. ... 

The use of 4-heteroarylmethylene- and 4-aryhnethylene-5(47/)-oxazolones as dienophiles in the Diels-Alder reaction has been recently reviewed. More recently the reactivity of the exocyclic double bond of 4-(alkoxymethylene)-5(4//)-oxazolones with several dienes has been assessed. Reaction of 4-(methoxymethyl-ene)-2-phenyl-5(477)-oxazolone and 1,3-butadiene requires the presence of Et2AlCl as a catalyst and even then the Diels-Alder cycloadduct is obtained in low yield. 

Two asymmetric Diels-Alder approaches to analogues like 830 and 831 have been described. In the hrst case, chiral catalysts were employed for an enantiose-lective Diels-Alder reaction, but unfortunately, none of the chiral catalysts showed any enantioselectivity upon analysis of the reaction mixtures. Alternatively, a chiral oxazolone 840 incorporating a menthyl carbonate at C-4 was prepared and used as a dienophile with cyclopentadiene. In this case, the exo/endo ratio was 60 40 but no signihcant endo or exo diastereoselectivity was obtained (Scheme 7.258). ° ... 

Olefinic dienophiles such as maleic anhydride and maleimide add to 4,6-dimethyl-2-pyrone to afford mono Diels-Alder adducts. These on irradiation afford, by a photo-decarboxylation, a diene which can be trapped by a dienophile to afford the products (311). - - Acetone-sensitized photoaddition of the oxazolones (312) to maleic and dimethyl maleic anhydride yields the adducts (313). - ... 

Scheme 28) <92JHC25l>. Unsaturated 5(4//)-oxazolones also serve as dienophiles. The (Z) isomer of 2-phenyl-4-benzylidene-5(4//)-oxazolone reacts with cyclopentadiene in the presence of aluminum chloride or titanium tetrachloride to afford a mixture of exo and endo cycloadducts <90S1114>. 

Particularly useful dienophiles are iV-acyl-oxazolones - the products are protected c/x-l,2-amino-alcohols. ... 

Considerable attention has been paid to the reactions of oxazoles with typical Diels-Alder alkene dienophiles. The adducts can be transformed into pyridines by different routes (section 5.15.1.4). Electron-releasing substituents on the oxazoles increase the rate of reaction 5-alkoxyoxazoles are comparable in reactivity to typical all-carbon dienes. Particularly useful dienophiles are AT-acyl-oxazolones - synthons for c/j-l,2-amino-alcohols. ... 

B3LYP/6-311G (PCM) calculations have shown that the 4 + 2-cycloaddition of cyclopentadiene and -/ -nitrostyrene in nitromethane solution proceeded in a stepwise mechanism. Sulfanyl-methylene-5(4//)-oxazolones and -sulfanyl-a-nitroacrylates have been used as dienophiles in the Diels-Alder reactions to synthesize norbornene/ane amino acid derivatives suitable for peptide synthesis. Organoammonium salts of chiral triamine catalyse the Diels-Alder reaction of a-(carbamoylthio)acroleins with acyclic 1,3-dienes to produce cyclohexenes with sulfur-containing chiral quaternary carbons.The helical-chiral hydrogen-donor... 

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